The ultimate goal of this proposal is to elucidate the novel antiviral mechanism of host BAG3; a newly discovered filovirus VP40 interacting protein that inhibits viral egress. Filoviruses (Ebola [EBOV] and Marburg [MARV]) are Category A, high-priority pathogens, for which there are no commercially available vaccines or therapeutic agents. Thus, exploratory and innovative approaches are warranted to better understand viral-host interactions that both promote and inhibit the transmission of these deadly viruses. Our laboratory focuses on the mechanisms by which filoviruses interact with and recruit host proteins to regulate the budding process. Here, we identified an unanticipated interaction between filovirus VP40 and the host protein BAG3. BAG3 is a stress-regulated, co-chaperone protein whose function is to ensure cell survival and protein homeostasis via Chaperone-Assisted Selective Autophagy (CASA). While BAG3 has been shown to positively influence entry and replication for some viruses, we found that the interaction of the PPxY motif of both EBOV and MARV VP40 with the single N-terminal WW-domain of BAG3 leads to an unexpected decrease in egress of both EBOV and MARV VP40 virus-like particles (VLPs), as well as VSV recombinant viruses that serve as functional EBOV surrogates, as they are genetically engineered to express either EBOV VP40 WT or mutant L-domains in place of the L-domain of VSV M. We hypothesize that BAG3 expression is induced following filovirus infection, and that BAG3 targets VP40 for removal and/or sequestration from the site of budding at the plasma membrane. We will use a rigorous multifaceted approach to interrogate the mechanisms by which BAG3 disrupts virus particle egress, thus providing new insight into the potential role of CASA as a novel and specific host defense strategy to counteract the ability of VP40 to promote efficient egress and spread of virus particles.